Internationally, the usage of so-called biometric characteristics is postulated increasingly, for example in the US-Visit and European programs.
The aim of these programs is to increase security, for example for airline passengers, and to recognize terrorists as early as possible.
For the acquisition of biometric data preferably characteristics of the face, of the iris and of the finger are used. Because of the high recognition rate and the invariability of the characteristics during the process of aging and because of the distinguishability even for uniovular twins, the characteristics of the finger are especially suited.
Especially for surveillance of persons at a border crossing, the biometric characteristics of a person demanding admittance have to be matched with existing, very large databases which can contain many millions of entries. In doing so, it has to be ensured that the biometric data being deposited in a fraud resistant way on a passport is in accordance with the entering person. As these characteristics have to be identified in a so-called 1:N search out of millions of sets of comparison information, in particular of criminal search databases, in case of finger characteristics a recognition problem of the following type arises:
The smaller the scanned surface of the finger which is to be identified the more inaccurate the results of the 1:N search are, because the number of characteristics, so-called minutiae, is not sufficient for a definite identification. Studies have shown that recognition methods using finger sensors which give only a simple impression of the finger reach a recognition rate of 80% to 85% only, because of the not entirely acquired finger surface. Such methods which are called flat finger impressions are predominantly used in civil areas. For protection against crime a recognition rate that low is insufficient.
Due to this, unrolled fingerprints have always been used in official criminology, where the person to be registered has to dye the fingers first and to roll them onto paper to be able to reproduce the entire surface of the finger from nail to nail.
Newer methods are known, too, where so-called live-scanners are utilized which replace the intricate dyeing, wherein the finger is rolled across a glass plate and the contrast due to a disabled total reflection arising thereof is used for optically imaging by means of a camera. With this method, it is possible to significantly increase the recognition rate of the 1:N identification in large databases to more than 98%.
With this method it is disadvantageous that the person to be registered has to roll the finger on an underlay. Normally, this results in problems with inexperienced users, wherein the fingerprints become smeared. Additionally, because of the different application pressure the fingers can deform differently or yield bad contrasts. Furthermore, it has been observed that, besides of a lack of contrast, in particular in case of dry skin grease marks are left which can also be recognized in a subsequent identification. To avoid this, the registration underlays consisting of glass have to be cleaned after each use as a general rule. Because a supervisor is required for an optimal accuracy, these methods cannot be used reasonably at automated control sites, for example so-called kiosks in the afore mentioned entry programs.
Systems are known that enable to image a finger touchlessly.
For example, it is known from EP 1 073 988 B1 and WO 99/56267 respectively to image a finger touchlessly, wherein the formation of the contrast is based upon exploiting features of polarized light. Thereby, the insufficient light efficiency turns out to be disadvantageous, because the polarization filter prevents an effective usage of the photons and therefore the efficiency factor is low. Furthermore, distortions caused by the imaging process are not compensated and result in the recognition of wrong minutiae very often, which is caused by shadowing to the dermal ridges in the utilized optical path of illumination.
Based on the touchless imaging principle, several techniques are generally known where the unrolled finger, i.e. the surface, can be imaged user-friendly. This includes, for example, DE 101 23 561 A1, DE 101 53 808 A1 and DE 101 03 622 A1.
In DE 101 23 561 A1, a method for identification of persons by means of a three-dimensional analysis of the fingertip is described. However, no method is given how to calculate the three-dimensional model and how to solve the problems of composing the single images to an overall image with sufficient accuracy.
If such a method is to be used for identification of persons at a country's borders, least criteria for the imaging quality, for example FBI Appendix F or similar criteria, have to be fulfilled, and it has to be ensured that the images taken from the dermal ridges are compatible with existing unrolled finger images. It is absolutely necessary to recognize persons on the basis of existing databases, in particular if these have been re-acquired with new optical methods.
In DE 101 03 622 A1, a method for imaging the surface of a finger is described, which reproduces the surface essentially by cylindrical imaging. However, it is disadvantageous that, on one hand, the finger is not illuminated uniformly due to the utilized principle and, on the other hand, that the imaging has very strong distortions, especially if the finger is not located exactly on the optical axis of the cylinder.
The DE 101 53 808 A1 describes another method for touchless optical creation of unrolled fingerprints, at which the image distortions shall be eliminated by creating a sequence of partial images whose overlapping areas are correlated as a structure of orientation by exploiting the pattern of capillary lines. Besides of the intricate realization of this arrangement, with such a method the demanded imaging quality is achievable not at all or only with largest efforts.
From US 2004/0008875 A1, a method for acquiring and processing three-dimensional fingerprints is known, wherein a finger which is arranged in contact with a transparent plate is scanned by means of at least two lasers, wherein the lasers sway along the finger, and a stereoscopic fingerprint image is created from overlapping scan lines and saved. The method exhibits all disadvantages of touching optical fingerprint methods. Additionally, because of the geometric arrangement of the finger on the plate, not the entire surface of the finger nor the entire finger-ball, respectively, can be imaged. Thus, the recognition rate is low. An accurate imaging method is not given in the document.
In the prior art, stereoscopic methods are inherently known. In particular, the Locus method is known, by which height deviations of an imaged body can be determined from stereo image pairs in relation to a geometric model which has to be provided.
In order to create stereo image pairs it is also known to perform a Hierarchical-Feature-Vector-Matching (HFVM) using calculations in image space.
In order to calibrate stereoscopic imaging devices, a method is known by which to determine the contortions and distortions of a stereoscopic imaging arrangement using a specimen.